U.S. patent number 4,859,277 [Application Number 07/189,572] was granted by the patent office on 1989-08-22 for method for measuring plasma properties in semiconductor processing.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Gabriel G. Barna, Demetre J. Economou.
United States Patent |
4,859,277 |
Barna , et al. |
August 22, 1989 |
Method for measuring plasma properties in semiconductor
processing
Abstract
An apparatus and method for measuring the concentration profile
of an active species across the surface of a semiconductor slice in
a plasma reactor is disclosed that permits uniformity of etch and
deposition across the surface of the semiconductor slice.
Inventors: |
Barna; Gabriel G. (Richardson,
TX), Economou; Demetre J. (Houston, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
22697907 |
Appl.
No.: |
07/189,572 |
Filed: |
May 3, 1988 |
Current U.S.
Class: |
438/7;
156/345.24; 118/50.1; 118/620; 118/712; 204/192.13; 204/192.33;
204/298.32; 427/8; 427/569; 438/9; 216/60 |
Current CPC
Class: |
H01J
37/32935 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); B44C 001/22 (); B05D 003/06 ();
H01L 021/306 (); C03C 015/00 () |
Field of
Search: |
;156/626,643,646,345
;204/192.33,192.13,298 ;356/437 ;427/8,38,39
;118/50.1,620,712,728 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Khoury, "Real-Time Etch Plasma Monitor System", IBM Tech. Discl.
Bulletin, vol. 25, No. 11A, Apr. 1983, pp. 5721-5723..
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Rogers; Joseph E. Comfort; James T.
Sharp; Melvin
Claims
What is claimed:
1. A method for determining the concentration profile of the active
gas species in a plasma reactor so as to allow the adjustment
thereof, comprising the steps of:
measuring the intensity of the emission of light from the plasma;
and
adjusting the concentration gradient of the active species
dependent upon the measured intensity of the light emission of the
plasma.
2. The method according to claim 1, wherein the surface of a
semiconductor slice is being altered by etched/deposition, the
measurement of the intensity of the emission of light from the
plasma is of a cylinder of light from the plasma parallel to the
surface of the semiconductor slice.
3. The method according to claim 2, wherein the measurement of the
intensity of the emission of the light from the plasma is done with
an optical train.
4. The method according to claim 3, wherein the optical train is
translated in a horizontal plane to profile the emission line of
light across the surface of the semiconductor slice.
5. The method according to claim 2, wherein the plasma is observed
through one or more diaphragms, an optical filter, and a focus lens
that directs the light into an optical fiber input of a
monochrometer.
6. The method according to claim 1, including the step of
introducing a gas in addition to the active gas species into the
plasma reactor to serve as an actinometer.
7. The method according to claim 6, wherein light emission of the
two gases are measured using a beam-splitter and two photodiodes
and two optical filter corresponding to the optical emission lines
of the two gases.
8. The method according to claim 6, wherein the introduced gas in
addition to the active gas species is argon.
9. The method according to claim 8, wherein the actinometer is
introduced in an amount of up to 6% mole fraction.
10. The method according to claim 1, wherein a small amount of
inert gas is introduced into the plasma and the intensity of the
light of the plasma and the intensity of the light of the inert gas
are compared.
11. A method for determining and controlling the concentration
profile of the active gas species in a plasma reactor comprising
the steps of:
introducing an actinometry gas into the plasma in the reactor;
measuring the intensity of light emitted by the gas species as
compared with the intensity of light emitted by the actinometry
gas; and
adjusting the concentration gradient of the active species gas
dependent upon the comparison of the species gas and the
actinometry gas.
12. The method according to claim 11, wherein the surface of a
semiconductor slice is being altered by etch/deposition, and the
measurement of the intensity of the emission of light from the
plasma is of a cylinder of light from the plasma parallel to the
surface of the semiconductor slice.
13. The method according to claim 12, wherein the measurement of
the intensity of the emission of the light from the plasma is done
with an optical train.
14. The method according to claim 13, wherein the measurement of
the optical train is translated in a horizontal plane to profile
the emission line of light across the surface of the semiconductor
slice.
15. The method according to claim 12, wherein the plasma is
observed through one or more diaphragms, an optical filter, and a
focus lens that directs the light into an optical fiber input of a
monochrometer.
16. The method according to claim 11, including the step of
introducing argon in addition to the active gas species into the
plasma reactor to serve as an actinometer.
17. The method according to claim 16, wherein light emission of the
two gases are measured using a beam-splitter and two photodiodes
and two optical filter corresponding to the optical emission lines
of the two gases.
18. The method according to claim 11, wherein the actinometer gas
is introduced in an amount of up to 6% mole fraction.
19. A method for uniformly etching or deposition across the surface
of a semiconductor slice in a plasma reactor utilizing one of more
specie gases comprising the steps of;
measuring the uniformity of the concentration profile of the active
species gas across the surface of the semiconductor gas; and
adjusting the concentration of the active species gas dependent
upon the measurement profile.
20. The method according to claim 19, wherein the surface of a
semiconductor slice is being etched, the measurement of the
intensity of the emission of light from the plasma is of a cylinder
of light from the plasma parallel to the surface of the
semiconductor slice.
21. The method according to claim 20, wherein the measurement of
the intensity of the emission of the light from the plasma is done
with an optical train.
22. The method according to claim 21, wherein the optical train is
translated in a horizontal plane to profile the emission line of
light across the surface of the semiconductor slice.
23. The method according to claim 20, wherein the plasma is
observed through one or more diaphragms, an optical filter, and a
focus lens that directs the light into an optical fiber input of a
monochrometer.
24. The method according to claim 19, including the step of
introducing a gas in addition to the active gas species into the
plasma reactor to serve as an actinometer.
25. The method according to claim 24, wherein light emission of the
two gases are measured using a beam-splitter and two photodiodes
and two optical filter corresponding to the optical emission lines
of the two gases.
26. The method according to claim 24, wherein the introduced gas in
addition to the active gas species is argon.
27. The method according to claim 24, wherein the actinometer is
introduced in an amount of up to 6% mole fraction.
28. The method according to claim 19, wherein a small amount of
inert gas is introduced into the plasma and the intensity of the
light of the plasma and the intensity of the light of the inert gas
are compared.
29. An apparatus for determining the concentration profile of an
active species across the surface of a semiconductor slice in a
plasma reactor, comprising an optical system for observing the
plasma emission within the plasma reactor, a monochrometer for
measuring the intensity of at least one specific emission line from
the plasma profiled across the surface of the semiconductor slice,
and means for adjusting the concentration profile across the
surface of the semiconductor slice to provide uniform
etch/deposition on the surface of the semiconductor slice.
30. The apparatus according to claim 29, wherein the optical system
includes one or more diaphragms, at least one filter, and a lens,
for directing the plasma emission from the plasma reactor to the
monochrometer.
31. The apparatus according to claim 29, wherein an actinometer gas
is introduced into the reactor, and including a beamsplitter, two
photodiodes and at least two filters to measure the emission lines
from the active species and the actinometer gas forming the plasma
in the reactor.
32. A method for determining the three dimensional concentration
profile of the active gas species in a plasma reactor so as to
allow the adjustment thereof, comprising the steps of:
measuring the intensity of the emission of light from the plasma;
and
adjusting the concentration gradient of tee active species
dependent upon the measured intensity of the light emission of the
plasma.
Description
FIELD OF THE INVENTION
This invention relates to semiconductor processing, and more
particularly to a technique for measuring plasma properties such as
the concentration profile of the active species across the surface
of a semiconductor slice during processing, including plasma etch
and deposition procedures.
BACKGROUND OF THE INVENTION
One of the problems in plasma etching and deposition is the
uniformity of the etch or deposition across the surface of the
slice. While it is generally understood that the fundamental plasma
properties, such as reactant concentrations, ion density/energy
profiles, and plasma sheath potentials, control the dependent
variables such as rate, uniformity and anisotropy of a process,
these properties are typically not monitored in a commercial plasma
reactor. The usual approach to controlling these dependent
variables is to monitor and control the independent variables such
as gas flows and pressures. They are then correlated to the
dependent variables via intuition or some empirical model. While
these approaches have been effective, the control of plasma
processes would be enhanced by a knowledge of the fundamental
plasma properties.
Basically, the only typical plasma characterization tool available
on all plasma reactors is an endpoint detection system. This is a
spectroscopic tool that looks at the intensity of the plasma at a
single wavelength that corresponds to the emission of a reactant or
product species in the process. A change in the intensity of this
signal indicates a change in the concentration of the specific
species, and hence indicates the completion of an etch process.
However, this method provides no indication of etching uniformly
nor of any of the fundamental plasma properties.
BRIEF DESCRIPTION OF THE INVENTION
The invention defines a new measurement technique which is
incorporated into a plasma reactor. The invention measures, and
allows control of the concentration profile of the active species
across the surface of a semiconductor slice being processed in the
plasma reactor. This parameter is only one of many that control the
uniformity of the reaction involved. The ability to measure this
parameter independently enhances the uniformity of plasma
reactions.
The method involves the measurement of the intensity of the
emission of a cylinder of light from the plasma, parallel to the
surface of the slice. This is done with an optical train. The
plasma is observed through one of more diaphragms, an optical
filter, and a focus lens that directs the light into an optical
fiber input to a monochrometer.
Alternatively, when using a second gas such as argon as an
actinometric gas, the monochrometer could be replaced with by a
beam-splitter and two photodiodes with filters corresponding to the
emissions lines of Ar.sup.* and X.sup.*. By translation of this
train in the horizontal plane, the intensity of the specific
emission line is profiled across the surface of the semiconductor
slice. Using actinometry, the mixing of a small percent of Ar into
the plasma, which measures the intensity of the X.sup.* species vs.
the Ar*, and the appropriate mathematical transformation (Abel
transform), the concentration profile of X.sup.* is determined
across the surface of the slice.
Using the method and apparatus of the invention, the concentration
profile of the species X.sup.* can be monitored and adjusted
through control of independent parameters such as pressure and
flow. Other parameters such as reactivity of the hardware
components peripheral to the slice and the plasma can also affect
the concentration profile.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a Parallel Plate plasma reactor used in the
invention;
FIG. 2 is a cross section of the radially symmetric glow take at a
selected parallel plane;
FIG. 3a is a plot of atomic oxygen mole fraction vs. radial
position in an empty reactor;
FIG. 3b is a plot of atomic oxygen mole fraction vs. radial
position in a loaded reactor;
FIG. 4 is a plot of atomic oxygen mole fraction vs. radial position
in a loaded reactor with a reactive film radius of 3.75 cm; FIG. 5
is a plot as in FIG. 4, but under different conditions; and FIG. 6
is a plot of atomic mole fraction vs. radial position in a loaded
reactor with two different reactive film radii.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1 illustrates a parallel plate plasma reactor 10 used in the
present invention. The parallel plate single-wafer etcher has, for
example, a 13.1 cm diameter, hard-anodized aluminum powered shower
head upper electrode 11 held at a distance of approximately 2.2 cm
from the 13.9 cm, diameter aluminum grounded lower electrode
12.
The temperature of both electrodes is controlled with a closed-loop
system, and the lower electrode temperature is monitored with an
embedded Fe-Constantan thermocouple. Gases are pumped into the
reactor by a two stage rotary vane pump, and the base pressure is
maintained at less than 1 mTorr. The chamber pressure and gas flow
rate are independently controlled by a closed loop system composed
of a pressure transducer, an exhaust throttle valve, and a
controller. The gas flow rate is regulated by mass flow
controllers. Extra dry oxygen and argon (as actinometer) are used
in the system. Power to the upper electrode 13 is at a frequency of
13.56 MHz. An automatic matching network is used to minimize the
reflected power to less than 1 % of the forward power. Both forward
and reflected power are monitored by a direction wattmeter. Light
from the plasma emission is collected through a quartz window 14
through a pair of iris diaphragms D to attain spatial resolution.
The light hv from the plasma is focused onto one end of an optical
fiber OF and is transmitted to the entrance slit of a monochrometer
MC having a 1200 groves/mm diffraction grating. The light is passed
through the exit slit and is detected by a photomultiplier tube PMT
driven by a photometer. A long pass filter F (>495 nm) is used
to avoid second-order interference in the light emission
spectrum.
The following is an analysis of an oxygen plasma system and is not
intended to limit the method of the invention to a single system,
but is used only by way of example. The purpose of the following
system is to illustrate an active species concentration profile and
to show the dependence on plasma operation conditions. Oxygen
plasma is used in the system because the glow discharge reactions
are better known as compared to other systems. A silver oxide
(Ag.sub.2 O) film is used as the reactive substrate. The film is
prepared by coating part of the lower electrode with a silver
paint. When the coating is exposed to an oxygen plasma, the organic
binder is burned and the remaining silver is turned into black
silver oxide. Silver oxide is known to be catalytic towards surface
recombination of atomic oxygen, the main etchant species in an
oxygen plasma. The silver oxide/oxygen plasma system is believed to
simulate etching of a thin film. Using a silver oxide film instead
of etchable material (e.g. polymer in the case of oxygen plasma)
has several advantages. First, the coupling of the plasma to ground
is nearly uniform as compared to possible non-uniformities of
plasma coupling through a silicon wafer resting on the electrode.
Secondly, contamination of the plasma by reaction products is
absent.
The surface reaction on the silver oxide is simply the
recombination of atomic oxygen to yield molecular oxygen. Thus, the
discharge is regarded as one in pure oxygen. Hence, data for pure
oxygen plasma is used for calculations, and any reactant
concentration gradient is due to differences in reactivity between
the Ag.sub.2 O film and the surrounding electrode surface.
Optical emission spectroscopy (OES) with an actinometer gas is
employed to obtain the reactant concentration profiles. In OES, the
intensity of light emitted by a species may be related to the
ground state concentration of that species. The pertinent reaction
may be written as ##STR1##
Reaction (Rl) is the excitation of species O by electron impact.
The excited species O.sup.* can decay by spontaneous emission (R2)
or by quenching upon collision with other species (R3).
If reaction (R3) can be neglected, the spontaneous emission
intensity is proportional to the ground state concentration
[O].
The above analysis assumes electron-impact excitation as the
dominant mechanism for producing O.sup.*. For example, a reaction
of the type
would invalidate actinometry if the corresponding O.sup.* -atom
were to emit at the wavelength of interest. However, in equation
(1), the so called excitation efficiency .eta..sub.e =k.sub.e
n.sub.e depends on the reactor operating conditions. In order to
account for this variation, a small amount of inert gas (the
actinometer) is used which has an excitation threshold and a cross
section similar to the species of interest. This way, although the
individual .eta..sub.e values for the actinometer and the species
of interest change with operating conditions, their ratio remains
almost constant. Hence by writing an equation similar to equation
(1) for the actinometer gas (for example Ar), and taking their
ratio results ##EQU1## where q is a proportionality constant and
[Ar] is the known concentration of the actinometer. By measuring
i.sub.o and i.sub.Ar at the appropriate wavelengths, the relative
change in [O] can be obtained. In the measurements of the present
example, a 8446 angstroms O-atom line and a 7540 angstroms Ar line
were used. A 5% mole fraction of the actinometer gas was used.
FIG. 2 shows a cross section of the radially symmetric glow taken
at a certain plane parallel to the electrodes (for example
z=z.sub.o). The optical train collects light from a small
cylindrical volume, as shown in the Figure. By translating the
optics parallel to the x-axis, light from different locations is
collected and intensity I(x,a.sub.o) measured. The latter may be
related to the local emission intensity i(r,z.sub.o) as ##EQU2##
where ##EQU3##
Equation (3) accounts for radiation trapping with a constant
absorption coefficients, .alpha.. for sufficiently small .alpha.,
equation (3) reduces to ##EQU4## The last integral is inverted by
applying an Abel transform to yield ##EQU5## where
I'(x,z.sub.o)=dI(x,z.sub.o)/dx. Equation (6) is numerically
integrated using the measured values of I(x,z.sub.o). In this
manner the local intensity profile is obtained.
The resulting profile (especially close to r=0) is found to be
sensitive to the scatter of the I(x,z.sub.o) experimental date. The
sensitivity is minimized by smoothing the data.
At a given axial position z=z.sub.o, radial emission intensity
profiles i(r,z.sub.o) are obtained for both O-atoms and Ar.
Equation (7) is then applied to find the ground state O-atom
concentration profile. ##EQU6## Note that since Ar is not reactive,
C.sub.Ar is independent of position.
FIGS. 3-6 illustrates experimental data as compared to ideal
condition or model predictions (solid lines). The experimental data
was obtained by spatially resolved optical emission spectroscopy.
The experimental conditions were as follows: Pressure was 1-3 torr;
power was 20-50 watts; gas flow rate was 25-100 sccm; diameter of
reactive surface was 7.5-10 cm,; and frequency was 13.56 MHz.
FIG. 3a shows the atomic oxygen mole fraction as a function of
radial position for three different values of power into the
plasma. FIG. 3a is the case of an empty reactor, there is no
--Ag.sub.2 O--film on the electrode surface. The concentration is
nearly uniform close to the reactor center and decreases
monotonically further away. The concentration is higher at higher
power due to increased etchant production.
A dramatic change in the concentration profile occurs, FIG. 3b ,
when part of the lower electrode is coated with Ag.sub.2 O in the
form of a concentric disk having a diameter of 75mm (loaded
reactor). A "dip" in the O-atom concentration profile occurs over
the reactive surface and large concentration gradients appear,
especially around the boundary between active and relatively inert
surfaces (for example, around 3.75 cm position). As power
increases, the reactant concentration gradients become steeper.
This is because at high power the reactant is produced faster that
it can diffuse. Further, the reactant concentration is
substantially lower in the case of a loaded reactor because of
increased reactant losses.
The effect of pressure is shown in FIG. 4. At lower pressure the
diffusivity increases (D.about.P.sup.-1) and concentration
gradients are smaller. However, the reactant concentration
decreases with decreasing pressure.
The effect of flow rate is shown in FIG. 5. A higher flow rate
results in better uniformity, but at the same time etch rate
deceases. The effect of surrounding the active surface by one with
similar reactivity is seen in FIG. 6, where the concentration
profiles resulting from two different Ag.sub.2 O coating diameters
are compared. The concentration profile is more uniform for larger
diameter coatings, at the expense of lower reactant concentration
due to increased loading.
Axial O-atom concentration gradients close to the substrate are
very week in the empty reactor implying negligible surface
reaction, however, large concentration gradients are observed in
the loaded reactor, implying fast surface reaction.
Etching uniformity improves if the electrode surrounding the wafer
is reactive against the etchant species (by surface recombination
reaction). This can be implied from FIG. 6, considering the "10 cm
radius" film to be a 7.5 radius film, with another 2.5 cm of
identical reactivity film surrounding it. The concentration profile
of the oxygen is made more uniform by this "external area of equal
reactivity". This result is in contrast to the Reinberg-type radial
flow reactor, where radial nonuniformities are common even if the
whole substrate electrode area has the same reactivity.
Reaction selectivity may be defined as ##EQU7##
When S<1, a "bullseye" film clearing pattern results. When
S>1, the inverse film clearing pattern results, i.e. etch rate
decreases monotonically from the wafer center to the wafer
periphery. The fact that etching is uniform when S=1 implies that
if the plasma radius equals the wafer radius, etching is uniform.
This is because there is no source of etchant species in the volume
beyond the wafer radius. A method of improving etch uniformity if
the plasma radius is greater than the wafer radius, is to cover the
electrode area surrounding the wafer with a material having
reactivity similar to that of the wafer. Such action however may
result in decreased etch rate due to loading.
* * * * *